The present invention relates to an adhesive. More particularly, the present invention relates to such an adhesive having one polymeric component adapted to provide adhesive properties and another polymeric component adapted to provide tackifying properties. More particularly still, the present invention relates to such an adhesive wherein both the adhesive and tackifier components are separately prepared by emulsion polymerization.
Aqueous collodial dispersions of polymers, which are referred to in the art as latexes, are generally known to be useful as adhesives. A wide variety of adhesive formulations have been prepared that are specially adapted for particular uses. While latexes generally provide good adhesion to various substrates, it is often desirable to include a secondary component, generally a low molecular weight emulsified resin which is employed in order to provide initial stickiness or tack to the adhesive formulation. Such a component is referred to as a tackifier. Certain prior art adhesive formulations have included a latex based adhesive component such as a latex of a styrene/butadiene copolymer, and conventional tackifiers such as rosin esters, or other suitable tackifier, see e.g. U.S. Pat. No. 4,189,419. The properties of the adhesive composition may be adjusted by appropriate selection of both the type and amounts of tackifier or adhesive component employed in order to provide a resulting composition having the desired properties.
However, such adhesive formulations have certain undesirable properties. For example, the process for dispersing the tackifying resin adds unnecessary complexity and additional cost to the formulation. In addition, such formulations often demonstrate insufficient coalescence between resin particles and latex particles, thereby necessitating using excessive amounts of tackifier resin. This problem has been previously disclosed in U.S. Pat. No. 4,445,959, Column 12. Also, because relatively high levels of surfactant are required to disperse the tackifying resin adhesive properties are often detrimentally affected. Finally, under some conditions of high shear such as may be encountered during application of adhesives these previously known adhesive formulations may lack sufficient stability.
Techniques are already known for the preparation of low molecular weight polymers via free-radical, emulsion polymerization processes. Typically, such techniques include initiation by organic peroxides or inorganic persulfate compounds, and the use of large quantities of a suitable chain transfer agent such as a halogen compound, a mercaptan compound, or the like.
Latex based adhesive components, especially copolymers of styrene and butadiene are similarly known. Suitable examples of such adhesive components are disclosed in U.S. Pat. No. 4,419,481. The teachings of which are incorporated herein by reference.
In GB No. 2,137,212, there is disclosed a tackified S/B polymer adhesive made by preparing an emulsion polymerized adhesive component in the presence of an emulsion of a synthetic tackifier resin or rosin derivative.
In U.S. Pat. No. 4,438,232, a latex adhesive resin is disclosed. In U.S. Pat. Nos. 4,013,824, 4,064,377 and 4,145,494, emulsion processes for preparing polymers with a mercaptan as a chain transfer agent are disclosed.
It would be desirable to prepare by emulsion polymerization a latex of an ultra low molecular weight polymer suitable for use as a tackifying resin which does not require dispersing a solid polymer in water by use of surfactant.
In addition, it would be desirable to provide a latex based adhesive formulation which does not require the presence of a conventional, emulsified tackifier resin.
Finally, it would be desirable to provide an adhesive formulation wherein both the adhesive component and the tackifier component are prepared entirely by emulsion polymerization.